Applicants hereby claim foreign priority under 35 U.S.C. xc2xa7119 from German Application No. 101 56 809.6 filed 20 Nov. 2001, the disclosure of which is herein incorporated by reference.
The invention concerns a method for the measurement of the concentration of a substance in a liquid, especially for blood sugar measurement, wherein the liquid to be measured is applied to the measuring field of a test strip, which measuring field is composed of a hydrophilic material, and the change of the optical reflectivity or transmissivity thereby effected in the area of the measuring field is captured.
In the case of a blood sugar measurement carried out by a patient himself or herself, the patient supplies a drop of blood to the test field of a test strip which is then optically measured, with a detector for example capturing the change in color of the measuring field, which change in color is evoked by the application of the blood to the test field. The signal given by the detector corresponds to an average value of the color change in the test field. A correct value is then only obtained if the amount of blood is sufficient to uniformly wet the entire measuring field. On the other hand, if the measuring field is only partially wetted, the measurement is falsified in that in the communication of the measured value areas are included in which practically no chemical reaction has taken place and accordingly no color change has occurred. By the time this fault is recognized in a customary measurement as a rule no subsequent dosing is any longer possible, because in the wetted portion of the measuring field the chemical reaction already has advanced too far and even with a further dropping of blood a uniform discoloration of the measuring field can no longer be obtained. Moreover, as rule on the part of the patient the small puncture wound out of which the blood drop has been pressed has again so far closed that no further blood escapes. The patient must therefore repeat the measurement, which for him or her is very unpleasant when one remembers that a patient, depending on circumstances, may have to carry out such measurement several times a day and each time has to stick himself or herself in a finger.
The invention has as its object the provision of a method of the aforegoing type in which the previously mentioned fault is timely recognized and can be overcome in the same measuring procedure.
This object is solved in accordance with the invention in that the measured value obtained after a pre-given time is compared with a reference value and in that an indication is made if a relationship of the measured value to the reference value exceeds a pre-given threshold value.
With a normal amount of liquid, that is a sufficient amount of liquid for carrying out the measurement correctly and a low content of the to be measured substance, after a given reaction time a minimum amount of discoloration, depending on the type of the test strip (corresponding to an upper reflectivity limit) is achieved. If the reflectivity lies above this limit value the measuring field is presumed to have not been sufficiently wetted with the investigated liquid, that is an under-dosing has occurred. Either more liquid must be redosed or the measurement must be repeated.
Basically the difference between the measured value and the reference value can be evaluated. Their ratio is however more advantageous since it reduces eventual disturbing factors. Understandably the ratio of the reference value to the measured value can also be evaluated with then an indication resulting if a threshold value is undershot.
In a preferred embodiment the measuring field is divided into at least two measuring areas which are separately measured, with each measured value being compared with a respectively associated reference value. In this way a non-uniform wetting of the measuring field can be captured.
The previously described methods can also be so carried out that first a first measuring area is measured and that a second measuring area is measured only when the relationship of the measured value and reference value for the first measuring area reaches or undershoots a pre-given threshold value.
The basic object of the invention can, in a method of the previously mentioned type, also be solved in that within a pre-given amount of time a plurality of measurements are carried out, that each measured value is compared with the measured value taken a given number of measuring steps previously, and that an indication is given if the relationship of the measured value and the comparison measured value exceeds a pre-given threshold value.
Within the reaction time in the case of normal courses of reaction, that is in the case of a sufficient dosing, the reflectivity curve steadily falls. Therefore, normally each measured value is smaller than the previously measured value. If within the reaction time a rise in the measured value takes place this indicates a bleaching of the measuring field because of a too small amount of the liquid to be investigated. Also in this case the measuring field can be divided into several measuring areas which are separately measured, in order to be able to test for an areawise uniform wetting of the measuring field.
Further advantageous embodiments of the invention are given in the further dependent claims.